1. Field Of The Invention
This invention relates to a method and apparatus for reducing noise disturbance in wide-band electrical signals. More particularly, this invention relates to a method and circuit arrangement for reducing unwanted noise in a television video signal using a process known as coring.
2. Description Of The Prior Art
Television video signals are frequently subject to having unwanted noise superimposed on them. One kind of unwanted noise, called textural noise, is caused by granularity in the devices that generate or store the video information. Examples of textural noise sources include: vidicon target irregularities, grain in the film used in tele-scene apparatus, magnetic tape irregularities, and flying spot scanner phosphor granularity patterns. Another kind of unwanted noise is scintillation noise including electronic thermal and shot noise.
A television video signal contains picture information over a broad band of frequencies. The large area brightness level information is contained in the low-frequency component of the signal, while the detail or sharp edge information is contained in the higher frequency component of the video signal. Textural and scintillation noise both possess a low-amplitude high-frequency characteristic when compared to the picture information. Therefore it is possible to improve the signal-to-noise ratio of a video signal by removing or suppressing the high-frequency low-amplitude portions of the signal. This is accomplished, of course, at the expense of the low-amplitude high-frequency portion of the picture information, which as a general rule can desirably be traded off for a reduction in noise. The removal of the low-amplitude excursion from the high-frequency portion of a signal is known in the television art as coring.
Known prior art systems for reducing unwanted noise in video signals by coring employ high pass filters to extract the high-frequency component from the video signal. One such device disclosed in U.S. Pat. No. 3,333,055 issued July 25, 1967, to Krause employs band pass filters to separate a television signal into low-frequency and high-frequency components. The high-frequency component of the signal is passed through a threshold device to inhibit any signal below a predetermined threshold value. The output of the threshold device is then combined with the low-frequency components of the television signal to provide a television signal with reduced noise.
Another approach to noise reduction in a video signal also employs a high pass filter to extract the high-frequency component of the signal. See U.S. Pat. No. 3,602,737, issued Aug. 31, 1971, to Radecke. In the device disclosed by Radecke, the high-frequency component of the signal is separated into parts above and below a given amplitude, and the low-amplitude portion of the high-frequency component of the signal is subtracted from the wide band signal to yield a noise suppressed wide band signal. The high-amplitude portion of the high-frequency component of the signal is added to the noise-suppressed wide band signal to partially compensate for the low-amplitude detail that was lost in the noise suppression step. A common feature of these known prior art systems for reducing noise in a video signal is their use of band pass filters to directly extract the high-frequency component of the signal prior to coring. It is a well known phenomenon of band pass filters that they undesirably disturb the phase relationships in signals that are passed through them. This phase distortion is particularly undesirable in the high-frequency component of a video signal.
One known device which confronts the problem of phase distortion in the high-frequency portion of the video signal is shown in U.S. Pat. No. 3,715,477 issued to Olson et al, on Feb. 6, 1973. The Olson patent discloses a video-signal noise-limiting apparatus wherein a video signal is separated into high and low frequency components by a comb filter having the advantage of a linear phase delay. The high-frequency component is cored to remove low-amplitude excursions therefrom and the cored high-frequency signal component is combined with the low-frequency component in suitable proportions to produce a video signal having an improved signal-to-noise ratio.
One disadvantage to the use of a comb filter to separate the video signal into high and low frequency components is the difficulty of achieving a relatively clean separation between the high and low frequency components of the signal. Another disadvantage is that, due to the nature of the comb filter, the high-low frequency separation occurs near the mid-frequency of the wide band signal, which is not the most desirable separation point for applications such as the removal of the effects of film grain from a video signal.